diff --git a/src/caosdb/common/models.py b/src/caosdb/common/models.py
index a3159821aa7b8ea7a4e1767e2f0895e751eb23a3..8a961a52cf4e3aad9eb13189afc6143fbc4b624b 100644
--- a/src/caosdb/common/models.py
+++ b/src/caosdb/common/models.py
@@ -2580,6 +2580,7 @@ class Messages(list):
 
         super().append(msg)
 
+    @staticmethod
     def _hash(t, c):
         return hash(str(t).lower() + (str(",") + str(c) if c is not None else ''))
     # end remove